Councilor at Large John R. Connolly raised the second most in July, $162,697. That left him with close to $700,000 in his campaign account, said spokesman Adam Webster.

But even after a robust month of harvesting money from donors, Connolly and Walsh still have substantially less in their campaign accounts than Daniel F. Conley. The Suffolk County district attorney for the past decade, Conley has amassed more than $1.2 million over the years, cash he now has at his disposal in the mayor’s race.

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Conley’s fund-raising cooled in July, when he raised just $140,180, leaving him third in the monthly totals reported by the campaigns. That was about $100,000 less than he had raised in May or June. Still, his campaign remained bullish.

Jessica Rinaldi For The Boston Globe

John R. Connolly

“We still had a great month, out-raised almost every other campaign, and have the largest cash on hand by far,” said Conley spokesman Mike Sherry. “I don’t think we have anything to be upset about.”

Twelve candidates are vying in the first open mayoral race in two decades. The two highest vote-getters in the Sept. 24 preliminary election face off in the general election Nov. 5.

Fund-raising is important because it builds campaign infrastructure and amplifies a candidate’s message, as each tries to capitalize on a core constituency, said Paul Watanabe, political science chairman at the University of Massachusetts Boston.

“Large injections of funds from organized interests do make a difference in a crowded field, and that’s what we’re seeing with Marty Walsh,” Watanabe said. “Often one looks at the unions as principally providing person power, and they do put troops on the ground. But they can also provide significant injections of money.”

The finance numbers reflect totals reported by campaign staffs to the Globe. Campaign finances are reported to the state Office of Campaign and Political Finance, but end-of-month numbers had not yet been posted online Wednesday.

Close behind in fund-raising were City Councilor Rob Consalvo, who generated $126,000 in July, according to spokesman Kevin Franck, and Councilor Mike Ross, who raised $123,202 in the month and now has $528,112 on hand, according to his campaign.

“Mike wants to use innovative ideas to solve Boston’s problems, and month after month, people want to invest in that vision,” said spokesman Josh Gee.

Until he launched his campaign, Walsh ran Boston Building Trades, which represents local unions of ironworkers, electricians, and other trade workers. He remains president of Laborers Local 223.

Wendy Maeda/ Globe Staff

Daniel F. Conley

Individuals can donate only $500 to a mayoral campaign. But unions can donate much more — up to $15,500 to a single candidate in a year — in addition to the support of individual members.

Walsh’s big donations include $15,000 from Boston Firefighters Union Local 718, $14,500 from Professional Fire Fighters of Massachusetts, and $15,000 from the International Union of Operating Engineers Local 4. His haul includes more than $25,000 from unions outside of Massachusetts.

Walsh also received $500 from developer Joseph F. Fallon, the owner of The Fallon Company, who lives in Belmont. And he picked up support from Mayor Thomas M. Menino’s former spokeswoman, Carole Brennan, now a public affairs consultant, and Thomas N. O’Brien, the former director of the Boston Redevelopment Authority.

Charlotte Golar Richie, a Dorchester Democrat who previously served in the state House of Representatives, collected $66,165
last month, said her campaign finance director, Kristin MacEachern. She expected the tally to grow overnight as final deposits were counted.

“The Charlotte Golar Richie committee is in striking distance of raising $200,000 in less than 90 days,” MacEachern said.

Richie, the only woman in the race, picked up the support of EMILY’s List, a political support group for female candidates, but has not yet shown an uptick in funds.

Bill Walczak, who cofounded the Codman Square Health Center, raised $40,138 in July, boosting his campaign cash on hand to $132,342, spokesman Wyatt Ronan said. But he was enjoying a fund-raising boost following his announcement that he opposes a casino in East Boston, a stance that contrasts with that of most of the field.

John F. Barros, former director of the Dudley Street Neighborhood Initiative, raised $39,578 in July, improving his cash on hand to $83,899, campaign manager Matt Patton said. Barros’s campaign built momentum through two new efforts, Patton said: a crowd-sourcing campaign that raised nearly $12,000 in 10 days via the Internet, and a voter engagement and registration effort at T stops, street corners, barber shops, and soccer fields.

“We know that folks have so much going on in their life that we want to meet them where they are,” Patton said. “If they could vote on the street corner, we’d have them do that, too.”

Councilor Felix G. Arroyo saw his fund-raising slip to $26,006 last month. His campaign still had $135,693 in cash on hand.

“It was a bit of a blow to him when he lost the bulk of his support from organized labor,” Watanabe said of Arroyo, a former union organizer. “I think from day one, his belief is, he’s going to try to out-organize the other candidates and not out-fund-raise them.”

Three candidates could not be reached for updates on their fund-raising.

However, state campaign finance reports show that as of mid-July, Councilor Charles C. Yancey had $45,247 cash on hand, Charles L. Clemons Jr. had $3,164, and David James Wyatt had $17.

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